The present invention relates to a ball valve which is made entirely of plastics material and comprises a tubular casing with connection pieces which are fastened to both its ends and which together form a through passage for a medium, a ball disposed in the through passage inside the casing and provided with a through opening, an operating means formed by a spindle inserted into an opening in the casing, cooperating with the ball, and provided with an operating grip to enable the ball to be turned so as to close or open the through passage.
Ball valves of this kind are intended to be installed in underground polyehtylene pipes, and must on the one hand function reliably and on the other hand be resistant to the penetration of dirt and moisture.
In known valves of this kind each of the connection pipes is usually provided with a seat for the valve and is fastened to the casing by butt welding. When the butt weld is made, the seats are axially displaced and moved towards one another. The problem thus arising is that the axial displacement during the butt welding must be such that on completion of the welding the ball is correctly enclosed between the seats. This procedure requires on the one hand accurate dimensioning and machining of the component parts which are to be welded together, and on the other hand a very accurately conducted welding process. In addition, changes may be caused by thermal expansion and contraction, so that the ball will no longer be enclosed under the correct pressure between the seats.